Ammonium nitrate contains no halogens, burns without smoke production, and is less detonable than other conventionally employed oxidizing materials. Ammonium nitrate is, other than ammonium perchlorate, one of the few readily available, inexpensive, inorganic oxidizers employed in certain energetic applications, thus making it an attractive candidate for such use.
However, the attractiveness of current commercially available ammonium nitrate in energetic compositions is tempered by several severely limiting drawbacks. Such drawbacks include an energetic performance significantly lower than comparable ammonium perchlorate-based compositions, low burning rates with relatively high pressure exponents compared to other oxidizer-containing compositions, and greater hygroscopicity (moisture sensitivity) than ammonium perchlorate.
Also, ammonium nitrate, exhibiting a very low crystal phase stability, passes through five distinct crystal phase changes in the temperature range of about -17.degree. C. to 169.degree. C. The most disadvantageous change or transition is the Phase IV .rarw..fwdarw.Phase III transition, occurring at about 32.3.degree. C. This Phase IV to Phase III transition is characterized by a significant irreversible increase in crystal volume. Thus, repeated cycling of ammonium nitrate-based energetic compositions through the Phase IV to Phase III transition temperature has been said to cause growth of the grain and destruction of grain integrity. The result is an increased porosity and loss in mechanical strength in an ammonium nitrate-based energetic composition.
Over the years, numerous efforts to stabilize ammonium nitrate to prevent or sufficiently suppress the Phase IV.rarw..fwdarw.Phase III transition have been made. In the agrochemical field a wide variety of ingredients have been tried at one time or another to prevent caking.
So too in the energetic composition field, where efforts to stabilize ammonium nitrate against Phase IV.rarw..fwdarw.Phase III transition have included combining ammonium nitrate with such materials as potassium nitrate, zinc oxide, magnesium oxide, potassium fluoride and, for instance, nickel oxide; as well as certain lithium, calcium, barium, and aluminum salts; and other metal salts of the nitrate anion. Further, compounds such as urea, ethylene diamine dinitrate, diethylene triaminetrinitrate, guanidinium nitrate, silicates, and, for instance, melamine have also been investigated as ammonium nitrate stabilizers. None has proven entirely satisfactory. For instance, with regard to potassium nitrate, at least about 9 wt. %, generally about 10-15 wt. %, is introduced into the ammonium nitrate crystal lattice to prevent transition up to about 121.degree. C. However, crystal stability is achieved, for instance, in propellant compositions only at the expense of increased pressure exponents, reduced energy performance, reduced density, and increased smoke, all of which make the use of potassium nitrate undesirable as a phase stabilizer. Ammonium nitrate stabilized with CuO or with about 3 wt. % NiO has been said to show little tendency to cake. However, such CuO and NiO-containing compositions present environmental hazards, sacrifice energetic performance, and present aging and processing problems in energetic formulations. Other metal salts have not sufficiently suppressed the Phase IV.rarw..fwdarw.Phase III transition.
Despite numerous efforts by numerous researchers to develop satisfactory phase-stabilized ammonium nitrate, no prior development has proven entirely adequate. The search for a solution has been complicated because the relative stabilizing effect of a given phase stabilizer is, in general, not predictable.